This invention relates to a method for the preparation of catalyst carriers. More particularly, it is concerned with a method for the preparation of silica or silica-alumina catalyst carriers.
It is well known that in the reaction in a heterogeneous system in which a solid catalyst with catalytic components supported on a carrier, there are involved diffusion process in the catalyst particles and absorption and desorption processes on the catalyst surfaces of the reactants and the products in addition to the surface chemical reaction process. In these processes, the reaction rate and the selectivity of the product are influenced by the structure of micropores in the catalyst carrier.
The structure of micropores in the catalyst carrier, therefore, should be adequate for diffusion of the reactants and the products in the catalyst particles.
As described above, structure of micropores in the catalyst carrier is an important element to consider in designing and optimumizing the catalyst for industrial use. However, technique for controlling the structure has not established.
This may be due to the fact that control of the structure of filled colloid particles, the precursor of the catalyst carrier, which is critical in determining the structure of micropores in the catalyst carrier, is difficult. The precursor of the catalyst carrier such as hydroxides and oxides, in general, exists in hydrated colloid state and agglomeration of the colloidal particles is influenced by too many factors, not only by the nature of the particles such as size, shape, distribution of the particle size, composition, form of bonding and thickness of the electric double layer but also by properties of the aqueous solution such as pH, nature and concentration of the electrolyte and viscosity and others.
Methods for improving the characteristics of the formed catalyst such as strength and porosity by adding an organic or inorganic fibrous substance to the catalyst material or the precursor thereof, or the carrier material or the precursor thereof followed by drying, baking and other operations are disclosed in publications such as Japanese Patent Laid Open Nos. 3304/1981 and 4404/1981. However, regarding the control of the structure and distribution of micropores in the porous substance, none of them refers to the possibility of controlling the structure and distribution of micropores in the porous substance by the nature of additives and the manner in which they are added and blended, although the fact that the openings are produced in the area occupied by the additive shrinked when dried or the one which disappeared when baked is referred to. When teflon is employed as the fibrous substance, the fibers are agglomerated and enlarged in the course of producing the alumina or silica-alumina catalyst carrier. Therefore, almost no effect is expected by the use of teflon for the control of the micropores.